body {
  font-size: 14px;
  margin: 0;
  box-sizing: border-box;
}
.sidebar {
  display: inline-block;
  width: 20%;
  box-sizing: border-box;
  vertical-align: top;
}
.sidebar li {
  cursor: pointer;
}
.sidebar li.selected {
  background-color: yellow;
}
.content {
  display: inline-block;
  width: 40%;
  padding: 5px;
  box-sizing: border-box;
}
.content textarea {
  width: 100%;
}
.preview {
  display: inline-block;
  width: 38%;
  height: 100%;
  padding: 5px;
  box-sizing: border-box;
  vertical-align: top;
}